Many solid-state devices require for their implementation the availability of thin layers of semiconductor of closely controlled thickness. A widely used method of obtaining such layers is that of epitaxial growth whereby single crystal material is deposited from either the liquid phase or vapor phase onto a suitable substrate. Examples of such devices are transferred electron diodes and Schottky gate Field Effect Transistors (FET) fabricated on epitaxial layers of GaAs. There exists a problem of thickness control during growth as well as that of maintaining a uniform thickness over the entire slice, which is essential if high device yields and uniformity of response are obtained. This problem is illustrated by considering the case of the FET which requires for optimum performance doping n.about.10.sup.17 cm.sup.-3 and layers only .about. 0.2 .mu.m in thickness.
An alternative to growing epitaxial layers of the required thickness is to grow an overly thick layer which is subsequently thinned. However, such techniques at best usually remove material uniformly so that initial nonuniformities in thickness are amplified when considered as a percentage of the final layer thickness. In addition, ambiguities resulting from difficulties in measuring thin layer thicknesses always exist.
Anodic thinning is one technique of thinning epitaxial material. Specifically it provides for automatic thinning to a thickness which is required for Schottky gate FET applications despite variations in layer thickness and substrate topography.
Devices prepared by this technique are not uniform over the entire surface but instead have hillocks or isolated regions of greater thickness dispersed randomly over the sample.